The present invention relates to a novel stilbeneamine derivative and an electrophotosensitive material using the same.
As an electrophotosensitive material for use in image forming apparatuses such as electrostatic copiers, laser beam printers, plain paper facsimiles and the like, a so-called organic electrophotosensitive material is widespread which comprises a combination of the following components:
a charge generating material for generating an electric charge (positive hole and electron) when exposed to light;
a charge transport material for transporting the generated electric charge; and
a binder resin.
The charge transport materials fall into two broad categories which include a positive-hole transport material for transporting positive holes of the electric charge, and an electron transport material for transporting electrons.
The organic electrophotosensitive material has an advantage over an inorganic electrophotosensitive material employing an inorganic semiconductor material in that the organic electrophotosensitive material is fabricated more easily at less production costs than the latter.
In addition, the organic electrophotosensitive material also has a merit of greater freedom of function design by virtue of a wide variety of options for materials including charge generating materials, charge transport materials, binder resins and the like.
In this connection, the organic electrophotosensitive materials have recently been widely used in the image forming apparatuses.
Unfortunately, however, the organic electrophotosensitive materials are inferior to the inorganic electrophotosensitive materials in photosensitivity and in electrical stability in the course of repeated image forming cycles, or in other words, repeatability.
With an aim at solving the above problem, intensive study has been made on the improvement of various materials for use in the organic electrophotosensitive materials. The charge transport material, in particular, is an important material affecting not only the sensitivity but also the repeatability of the electrophotosensitive material.
Japanese Laid-open Patent Publication JP 02-12256 A (1990) discloses a charge transport material of stilbeneamine derivative represented by a formula (2): 
wherein Ar1 denotes an aryl group or heterocyclic group, each optionally having a substituent; Ar2 and Ar3 are the same or different, each denoting an arylene group; and R1 and R2 each denote an alkyl group, aralkyl group or aryl group, each optionally having a substituent; provided that Ar3 and R1 or Ar3, R1 and R2 may be combined together to form a heterocycle. Incidentally, the patent publication cites quinolyl, carbazolyl, oxazolyl and imidazolyl as the heterocyclic group represented by Ar1.
Alternatively, Japanese Laid-open Patent Publication JP 58-123542 A (1983) discloses a stilbeneamine derivative which is represented by a formula (3): 
wherein X1 and X2 are the same or different, each denoting a phenyl group optionally having a substituent; X3 denotes an arylene group optionally having a substituent; and X4 denotes an aryl group, 2-furil group, or 2-thienyl group, each optionally having a substituent.
There is further disclosed in Japanese Laid-open Patent publication JP 09-134021 A (1997) a stilbeneamine derivative represented by a formula (4): 
wherein Ra, Rb, Rc, Rd and Re each denote a hydrogen atom, halogen atom, alkyl group, aryl group or alkoxy group; and x denotes an integer of 0 to 2.
However, the inventors have examined the stilbeneamine derivatives disclosed in the above official gazettes to find that all these compounds suffer the following problems:
that they still fall short of achieving sufficient compatibility with the binder resin to be uniformly dispersed in the photosensitive layer, thus suffering a low tendency of producing electric charge transfer;
that they are still short of electric charge transferring capability;
that they suffer insufficient in electrical, physical and chemical stabilities; and
that as a consequence, the electrophotosensitive materials comprising these compounds are not only sufficiently improved in photosensitivity but also suffer poor repeatability.
It is an object of the invention to provide a novel stilbeneamine derivative which has a larger electric charge mobility than the prior-art compounds, thus featuring excellent electric charge transferring capability and which is also excellent in compatibility with the binder resin and in stability.
It is another object of the invention to provide an electrophotosensitive material using the above stilbeneamine derivative and featuring excellent sensitivity and repeatability.
For achieving the above objects, the inventors have conducted analysis on the structure of a stilbeneamine derivative represented by a formula (1): 
wherein R1 denotes an alkyl group, alkoxy group, aryl group, aralkyl group, halogen atom or hydrogen atom; Ar1 and Ar2 are the same or different, each denoting an aryl group; Ar3 denotes an arylene group; and Ar4 denotes an arylene group or divalent heterocyclic group.
Finally, the inventors have discovered that the above objects can be achieved by limiting In, in the formula to 1 when a terminal thiophene ring of the stilbeneamine derivative is 2-thienyl or by limiting xe2x80x98nxe2x80x99 to 0 or 1 when the terminal thiophene ring is 3-thienyl.
That is, the stilbeneamine derivative of the invention is represented by the formula (1):
wherein R1 denotes an alkyl group, alkoxy group, aryl group, aralkyl group, halogen atom or hydrogen atom; Ar1 and Ar2 are the same or different, each denoting an aryl group; Ar3 denotes an arylene group; and Ar4 denotes an arylene group or divalent heterocyclic group; provided that when a thiophene ring in the formula is 2-thienyl, xe2x80x98nxe2x80x99 is 1 whereas when the thiophene ring is 3-thienyl, xe2x80x98nxe2x80x99 is 0 or 1.
The stilbeneamine derivative represented by the formula (1) is absolutely a novel compound which is not specifically described in any of the above official gazettes.
If the stilbeneamine derivative of the formula (1) is compared with a compound represented by the formula (2) and defined as Ar1 being an aryl group such as a phenyl group, the former is more improved in compatibility with the binder resin than the latter. This is because the thienyl group has a higher degree of asymmetry in terms of molecular structure and electrical property than the aryl group.
The thienyl group has a higher electron donatability than the aryl group and the other heterocyclic groups (such as the aforesaid quinolyl, carbazolyl, oxazolyl, imidazolyl and the like). Thus, the stilbeneamine derivative of the formula (1) is superior to the compounds of the formula (2) in electric charge transferring capability, as well.
If, out of the stilbeneamine derivatives of the formula (1), a compound having 3-thienyl group as the thiophene ring and defined as xe2x80x98nxe2x80x99 being 0 is compared with a compound represented by the formula (3) and having 2-thienyl group as X4 or a compound represented by the formula (4) and defined as xe2x80x98xxe2x80x99 being 0, the former has a larger ionization potential than the latter ones. Thus, the former stilbeneamine derivative is particularly more improved in electrical stability than the latter compounds.
If, out of the stilbeneamine derivatives of the formula (1), a compound having 2-thienyl group or 3-thienyl group as the thiophene ring and defined as xe2x80x98nxe2x80x99 being 1 is compared with the compound of the formula (3), the former has a larger spread of xcfx80-electron conjugated system than the latter because of the presence of xe2x80x94CHxe2x95x90CHxe2x80x94Ar4xe2x80x94 at the molecular center. Thus, the former stilbeneamine derivative is superior to the compound of the formula (3) in electric charge transferring capability. Furthermore, the former stilbeneamine derivative is more excellent in stability than the latter, because of the greater molecular weight of the basic skeleton.
If, out of the stilbeneamine derivatives of the formula (1), the compound having 2-thienyl group or 3-thienyl group as the thiophene ring and defined as xe2x80x98nxe2x80x99 being 1 is compared with a compound represented by the formula (4) and defined as xe2x80x98xxe2x80x99 being 1 or 2, the former has a larger spread of xcfx80-electron conjugated system than the latter. Thus, the former stilbeneamine derivative is superior to the latter compounds in electric charge transferring capability. Furthermore, the former stilbeneamine derivative is more excellent in stability than the latter, because of the greater molecular weight of the basic skeleton.
When all these facts considered, the inventive stilbeneamine derivative represented by the formula (1) is superior to the prior-art compounds represented by the formulas (2) to(4) incompatibility with binder resin, electric charge transferring capability and stability.
The electrophotosensitive material according to the invention comprises a photosensitive layer formed on a conductive substrate and containing the stilbeneamine derivative of the formula (1) as a hole transport material.
The electrophotosensitive material of the invention comprises the photosensitive layer containing therein, as the hole transport material, the stilbeneamine derivative of the formula (1) which, as mentioned supra, is excellent in electrical charge transferring capability and compatibility with the binder resin. Accordingly, the inventive electrophotosensitive material has a great rate of transferring electric charges (holes) generated in the charge generating material or a large electric charge mobility, thus featuring high photosensitivity when exposed to light. The large electric charge mobility contributes to a great drop of residual potential after light exposure. As combined results of this feature and the high stability of the stilbeneamine derivative, the electrophotosensitive material of the invention has an excellent repeatability.